Solid state imagers typically consist of an array of thousands or even millions of pixel cells organized into rows and columns. Each pixel cell contains a photosensor (e.g., a photodiode) that is configured to produce an electrical signal corresponding to the intensity of light impinging on it when an image of a scene is focused on the array by one or more lenses. Each signal can be converted to a digital pixel value (i.e., a number associated with a pixel cell), usually with small numbers representing lower light intensity (i.e., darker areas of the scene) and larger numbers representing higher light intensity (i.e., brighter areas of the scene). A digital image can be formed by organizing the pixel values into rows and columns corresponding to the arrangement of their respective pixel cells in the pixel array. The digital image can be stored in a memory, displayed on a monitor, manipulated by software, printed, or otherwise used to provide information about the scene.
Several kinds of imagers are generally known. Complementary metal-oxide-semiconductor (“CMOS”) imagers and charge coupled device (“CCD”) imagers are among the most common. CMOS imagers are discussed, for example, in U.S. Pat. No. 6,140,630, U.S. Pat. No. 6,376,868, U.S. Pat. No. 6,310,366, U.S. Pat. No. 6,326,652, U.S. Pat. No. 6,204,524, and U.S. Pat. No. 6,333,205, all assigned to Micron Technology, Inc.
For an imager to capture a color image, its pixel cells must be able to separately detect values of colored light, for example, red (R) light, green (G) light, and blue (B) light. A color filter array is typically placed in front of the array of pixel cells so each pixel cell measures only light of the color of its respective filter. The most common type of color filter array, often referred to as a “Bayer color filter array,” is described in U.S. Pat. No. 3,971,065. Bayer color filter arrays consist of alternating red and green filters in a first row and alternating green and blue filters in a second row. This pattern is repeated throughout the array. Thus, in an imager employing a Bayer color filter array, one-half of the pixel cells are sensitive to green light, one-quarter are sensitive to red light, and the remaining one-quarter are sensitive to blue light.
To provide a color image, however, red, green, and blue values are required for every pixel. Thus, the two “missing” color values at each pixel location must be estimated, usually by reference to other nearby pixel values. This estimation process is often refined to as “demosaicing.” Several demosaicing techniques are known in the art. For example, U.S. Pub. No. 2007/0153106 and U.S. patent application Ser. No. 11/873,423 (not yet published), both assigned to Micron Technology, Inc., disclose two demosaicing techniques.
After demosaicing, three or four color values are typically associated with each pixel, depending on the color space used by the imager or processing software. RGB and YCbCr are two common color spaces. In the RGB color space, also referred to as the “Bayer domain,” a red value (R), a blue value (B), and a green value (G) are associated with each pixel. In the YCbCr color space, a luminance (i.e., brightness) value (Y) and two chrominance (i.e., color) values (Cb and Cr) are associated with each pixel.
Sometimes color pixel values do not accurately reflect the true color in a corresponding part of a scene. For example, a pixel corresponding to the edge of a white region in a scene might appear colored rather than white. Digital image processing ran be used to correct such errors, which are also referred to as “color artifacts.” Aliasing artifacts are one common kind of color artifact. Aliasing occurs when detail in a scene exceeds the sampling frequency of the imager, for example, when the lines of detail in an image exceed the number of rows of pixels in the pixel array of an imager. Aliasing can result in color artifacts along edges of details in an image and especially, though not exclusively, along edges involving an abrupt black-to-white transition. The tendency of lenses to refract different wavelengths of light differently is another common cause of color artifacts. Color artifacts may also appear as isolated colored “dots,” sometimes comprising only a single wrongly-colored pixel, in an image. These isolated colored dots can be caused, for example, by a defect in the imager or an error during demosaicing.
Correcting the root causes of color artifacts requires additional hardware components or substitution of higher-quality hardware components, such as, for example, a pixel array with more pixels or lenses that refract varying wavelengths of light more evenly. These solutions are often impractical, particularly in low-cost imagers. Therefore, a less expensive method for correcting color artifacts in digital images, particularly one which does not require additional or higher-quality hardware, is desirable.